Electric discharge device and electrode therefor



Sept. 7, 1943.; E M- 2,329,126

ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Filed Sept. 1'1. 1941 lnven tor': v Eu ene emmers,

Patented Sept. 7, 1943 ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Eugene Lemmers, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application September 17, 1941, Serial No. 411,209

6 Claims.

This invention relates to electric discharge devices and electrodes therefor, and more particularly to cathode electrodes for discharges in ionizable atmospheres of gas or vapor, or bothas in various types of lamps containing vaporizable metal like mercury, sodium, or cadmium, and starting gas such as argon or neon, or mixtures of such rare gases. The invention is also concerned with the starting of such discharges. The invention may be appliedto discharge devices producing useful radiations for various purposes,

such as germicidal tubes or lamps, and lamps used industrially for irradiating or treating many substances and products, as well as lamps for lighting. I have hereinafter explained the invention with particular reference to its use in a low pressure positive column-discharge device such as a mercury, sodium, or cadmium lamp. Among the advantages that may be realized with my invention may be mentioned easy starting of discharge devices equipped with my electrode; shielding of envelope Walls against deposit thereon of material evaporated or sputtered from emissive cathode areas; and prolongation of the emissive life of a cathode electrode.

In one preferred form, my novel cathode may comprise a grid-like openwork electrode whose component members surround a common space and emit electrons at their .sides toward said space, but are not (in the early life of the device, at least) substantially emissive outward, from their out-turned sides. For starting the discharge, the cathode may be specially heated to render it emissive, preferably by passing current through it. The cathode is here shown as a substantially helical coil of metal wire, such as tungsten, molybdenum, etc., open between its convolutions, but initially emissive only from their inner sides. This cathode coil may be either a simple coil or a compound or multiple coilsuch as a coiled-coil, a triple coil, etc., according to the resistance and other characteristics. desired .cathode convolutions being shown ln.d0t-andphere in the discharge device, as hereinafter described, the compound type of coil that is preferred to give the necessary resistance also lends itself to carrying the amount of activating material that is required on the inner sides of its main ormajor convolutions, without need for.

any external coating on these main convolutions.

With an emissive cathode coil or grid for the purposes of my invention, there may be associated an auxiliary plate-like member of appreciable surface area, which may perform anode functions on'positive A. C. half-cycles, protect the envelope walls against deposits of material from the coil, and contribute to electron emission during the later life of the discharge device.

Various other features and advantages of the invention will appear from the following description of species or forms of embodiment, and from the drawing.

In the drawing, Fig. 1 is a diagrammatic general view of a discharge device such as a sodium lamp equipped with one form of cathode electrode, including a wiring. diagram of suitable circuitconnections; and Fig. 2 is a partial longitudinal sectional view of one of the cathodes shown in Fig. 1, on a larger scale, some of the dash outline merely.

Fig. 3'is a view similar to Fig. 1 illustrating modified electrode structure, on a smaller scale; Fig. 4 is a view at right angles to Fig. 3, taken as indicated by the line and arrows 4-4 in Fig. 3; and Fig. 5 is a diagrammatic view of a lamp equipped with electrode structures such as shown in Figs. 3 and 4. Fig. 6 is a view similar to Fig. 4 illustrating a further modification of the structure shown in Figs.3 and 4.

Fig. 1 shows a sodium vapor lamp L comprising a vitreous, tubular, round-ended envelope or bulb [0 with cathodes C, C mounted in its opposite ends. The envelope 10 may contain the usual low-pressure atmosphere of starting gas, such as neon at a pressure of 3 mm. of mercury, as well as a surplus supply of metallic sodium exceeding the amount that will vaporize during operation of the lamp L (as indicated by the droplet l2), and may have the usual special lining resistant to hot sodium vapor, as indicated at I3. The lamp L and its electrical connections are only diagrammatically represented in Fig. 1, and may in practice correspond to the usual construction of sodium vapor lamps-except that cathodes C, C embodying my present invention are indicated, and that the starting arrangements are prefer ably different from those of the ordinary sodium lamp-both in dispensing with the usual auxiliary starting electrode, and in other respects. For operation of the lamp L, the cathodes C, C are shown connected as usual across the secondary. circuit p oi? a step-down transformer T (of leakage reactance type) whose primary is connected across an ordinary A. C. power line P;

while for starting, an electrode-heating circuit H is shown connected across the circuit p through the cathode coils C, C, with a starting switch l8 of any suitable type (either manual or automatic) interposed therein. It will, of course, be understood that. the transformer T serves the purpose of a current-limiting ballast to keep the lamp from running away, as.we'll as of a starting inductance;

Each cathode C may comprise a thermionic preferably of refractory metal such as tungsten or molybdenum, and shown in Fig. 2 as consisting of a helical wire coil whose several convolutions form the component gridmembers 2|. To enable the device L to start without any auxiliary starting electrode, each-of the cathodes C, C should preferably be of such electrical resistance that the voltage required to heat it to electronemisslve temperature suillces to ionize the starting atmosphere in the'envelope Ill; and, in order to give the improved action which I have in view, the cathodic emissivity should be predominantly internal, initially. However, it is to be understood that the starting and internal emissivity features are separately useful, so that either can be used for certain purposes without the other.

The general mode of operation in starting the lamp L with this arrangement'is that while the electrode of, openwork or grid-like character,

power circuit P is energized, for example, from a 110 volt A. C. source, the switch I8 i closed to permit flow of current through the circuit H and the cathode coils in series therein for a suflicient length of time to preheat the cathodes C, C to an adequate emissive temperature. The voltage through each of the coils under this condition being suflicient to ionize the starting gas in the envelope [IL-say about 13 volts R. M. S. or more (corresponding to about 18 volts on the A. C.

' peaks) for a device L with a neon atmosphere of about 3 mm. pressure-the cathodes C, C heat up to emissive temperature in a few seconds, so that an isolated glow surrounds each of them. A few seconds later, a positive column discharge strikes throughthe length of the lamp L between the two cathodes C, C, and the main arc is thus established. The starting switch l8 may be opened to assure striking of the main discharge or are in this manner; but ordinarily this are strikes even before the switch I! is opened.

Thus starting may be accomplished on the normal operating voltage across the secondary circuit 3:

I of the transformer T, amounting usually to some 26-28 volts, more or less.

As shown in Fig. 2, the wire coil interiorly emissive by activating material 22 (partly indicated by stipDling) on the inner sides is rendered tween the crosswise extending coil and the adcomes more or less nearly dry, and releasingthe coil and'allowing itsconvolutions to spring apart and open up before finally activating itin the usual manner.

Cathodes c. 0 suitable for an ordinary sodiumvapor lamp and operable as above described may preferablybe of triple coilconstruction, and may be fabricated as follows:

Tungsten wire of 4 mil diameter may be wound 150 turns per inch on a tungsten wire of 16 mil size. The 16 mil wire with its winding of 4 mil wire may then be wound 25 turns per inch on a mandrel of molybdenum wire of 40 mil size; and this, in turn, may be wound 10 turnsper inch on a steel mandrel? inch in diameter. The two relativel large mandrels may be removed, as by withdrawing the steel one and dissolving out the other with ordinary molybdenum solvent, consisting of a mixture of nitric and sulphuric acids. The cathodes C, C may each consist of an 8- tum i inch compound coil thus fabricated, with portions 23, 23 of the intermediate coil extending perpendicular to the main coil axis to serve as current lead connections. For activating such a compound coil, it is the mainconvolutions 2| that are coated with the activating material 22' on their inner sides.

In the operation of a device with a pair of my cathodes C, C, such as shown in Fig. 1, it is found thatduring positive'A. C. half-cycles the unactivated exteriors of the cathodes C, C perform anode functions to the partial exclusion, at least, of the activated internal surfaces, thus protectingthe latter from electron bombardment without any need for the auxiliary anodes usually provided with activated cathodes of A. C. low pressure discharge devices. themselves also shield and protect the walls of the envelope I0 against deposits of material sputtered or evaporated from the activated inside areas of the grids. Indeed, activating material that deposits on the juxtaposed sides and on the exteriors of the'heated cathode members 2| continues to serve its intended function of electron emission, thus very considerably prolonging the useful emissive life of the cathodes C, 0.

Figs. 3, 4 and 5 illustrate a. modification in which a cathode coil has associated therewith auxiliary plate-like means 25 of substantial area. As shown in Figs. 3 and 5, the plate "associated with each coil is arranged to extend across the end of the tubular lamp envelope l0 perpendicular to the longitudinal axis of the envelope, be-

Jacent end wall of the envelope; and the plate extends between the current lead connections 23, 23 and may be mechanically attached and electricall connected at 25 '(as by welding) to either one of the connections 23, 23-though- I preferably to that to which the discharge circuit p is connected-but is not of course, electrically connected to both leads 23, 23. Each plate 25 may be of substantially the same lengthas the associated coil, and of a width equal to about 1% times to twice the outside coil diameter, and may be spaced from the coil 9. distance of about half the coil diameter or less. In Figs. 3 and 4' and 5, the plate 25 is shown flat, while in Fig. 6

it has the form of a round-bottomed trough with top flanges 21, 21 projecting from its edges.

The plate 25 not only serves as anode during positive A. C. half-cycles, but collects deposit: of activating material sputtered from the coil and shields the end wall of the lamp from such deposits, as well as from the direct heat of the The cathode grids cathode and 01' the discharge. Eventually, it also contributes to electron emission, by virtue of its deposits of activation material and of the heat which it receives from the cathode. Thus a cathode C activated only internally as in Fig. 2

2. A thermionic cathode for an electric discharge invan ionizable atmosphere comprising a and equipped with a plate 25 has three stages of emissive life: first, emission only from the initial activating coatings 22 on the insides of the coil convolutions 2|; second, emission from deposits of activating material on the adjacent sides and the'outsides of the convolutions; third,

such as a sodium or cadmium lamp, for instance plates 25, 25 located between the cathodes C,

' C and the adjacent end walls of the lamp envelope it may be used for another important purpose: i. e., to prevent excessive heat radiatedfrom. the cathode from heating the ends of the envelope id hotter than its mid-portion, which would have the result that the metal would condense and solidify mainly on the portion of the envelope wall between the plates 25, 25, where it would obscure the envelope. For this purpose, plates 25, 25 thus arranged may be found useful with cathodes that are activated externally as well as internallyalthough perhaps to a lessdegree, since such over-all emissive cathodes may aflord the necessary emission of electrons from their proportionately greater areas of emissive surface without being so large as my internally activated cathodes C, C, or giving out so much heat. a 'What I claim as new and desire to secure by Letters Patent of the United States is: I

l. A thermionic cathode for an electric discharge in an ionizable atmosphere consisting es- I sentially of an open grid electrode composed of members surrounding a common spaceand initially emissive only from their sides toward said space, the resistance of said electrode being such that the voltage required to pass therethrough suflicient'current to heat it to a temperature of emission also suflices to ionize the atmosphere.

discharge sion, but open between its convolutions and initially unactivated at its exterior, wherein the resistance of the cathode coil is such that the voltage therethrough required to'heat it to a temperature of internal emission suffices to ionize the discharge atmosphere. p 1

3. A thermionic cathode for an electric dischargein an ionizableatmosphere comprising a compound helical coil having such resistance that the voltage therethrough required to heat it to a temperature of emission sufiices to ionize the discharge atmosphere, and coated with activating material on the inner sides of its main convolutions, but open between said main convolutions and initially unactivated at its exterior.

4. The combination with a, tubular envelope for an ionizable discharge atmosphere and a cathode coil therein extending crosswise thereof adjacent an end wall of said envelope, said coil being internally activated for electron emission, but open between its convolutions and initially unactivated at its exterior, ofa conductive shield and electrode plate extending crosswise of said envelope between said cathode and said envelope end wall, and electrically connected to a current connection of the cathode. v

5. An electric discharge device comprising a tubular envielope containing a normally solid ionizable working substance, cathode coils therein extending crosswise thereof adjacent the end walls of said envelope, said coils being internally activated for electron emission, but open between their convolutions and'initially unactivated at their exterior, and conductive shield and electrode plates extending crosswise of said envelope between said cathodes and the envelope end walls, and electrically connected to current connections of said cathodes.

6. A- thermionic cathode for an electric discharge in an ionizable atmosphere comprising an open-grid electrode composed. of members surrounding a common space and initially electron emissive only from its interior, the resistance of said electrode being such that the voltage required to pass therethrough suificient current to heat it to a temperature of emission also suffices to ionize the discharge atmosphere. EUGENE LEMMERS. 

